Health and Ecological Effects

Cyanobacterial blooms in the United States have been associated with the death of wildlife and domestic animals. They may pose a risk to human health through the exposure to contaminated freshwater, the ingestion of contaminated drinking water, or the consumption of contaminated fish or shellfish. It is also clear that cyanobacteria pose a potential risk to aquatic ecosystems when present in large quantities as their decomposition causes excessive oxygen consumption, which leads to an increased mortality rate in local populations due to low oxygen levels (hypoxia).

Photo of lake
Cyanobacteria bloom at Grand Lake St. Marys, Ohio, 2010. Photo by Ohio EPA.

How are humans exposed to cyanobacteria and cyanotoxins?

The most common exposures to cyanobacteria and their toxins are believed to occur during recreational activities via oral, dermal, and inhalation routes. Oral exposure may occur from accidental or deliberate ingestion of contaminated water. Dermal exposure may occur by direct contact of exposed parts of the body to water containing cyanobacteria cells. Inhalation may occur through the aspiration of water containing cyanobacteria cells and their toxins. Wind-driven currents may cause buoyant cyanobacterial blooms to amass on shorelines. These accumulations of cyanobacteria cells are orders of magnitude larger than blooms in open waters, thus presenting a greater risk to human and animal health. Cyanobacteria cells can also accumulate in bathing suits, particularly diving suits. When the algal cells that have accumulated break, the wearer's skin is exposed to the toxins thereby resulting in the manifestation of exposure symptoms despite the absence of direct contact between the wearer’s skin and the contaminated water.

Other major routes of human exposure are through ingestion of cyanotoxin-contaminated drinking water, inhalation while showering, dietary intake via consumption of cyanotoxins in contaminated foods and algal dietary supplements, and exposure from water used in medical treatments (e.g., medical dialysis). The consumption of fish flesh (muscle) is usually considered safe but there are fish species and fish organs, especially the liver, and stomach/intestinal contents which may contain considerable amounts of cyanotoxins. Consumption of mussels and clams collected during cyanobacterial blooms or immediately after blooms should be avoided. Generally, hepatotoxic microcystins and nodularins are more common than the neurotoxins in aquatic animals including mussels, clams, crab larvae, prawns, crayfish and zooplankton, causing hepatotoxic effects in the fish and the accumulation of toxins in their organs. Consumption of contaminated shellfish and fish with cyanotoxins can lead to impacts on the liver and the nervous system.

What health risks do humans face as a result of exposure to cyanotoxins?

Adverse health outcomes from exposure to cyanotoxins may range from a mild skin rash to serious illness or death. Acute illnesses caused by exposure to cyanotoxins have been reported. The table below summarizes the health effects caused by the most common toxin producing cyanobacteria.

The Primary Cyanotoxins and their Health Effects

Cyanotoxins Acute Health Effects in Humans Most common cyanobacteria producing toxin
 Microcystin-LR Abdominal pain, Headache, Sore throat, Vomiting and nausea, Dry cough, Diarrhea, Blistering around the mouth, and Pneumonia Microcystis, Anabaena, Nodularia, Planktothrix, Fischerella, Nostoc, Oscillatoria, and Gloeotrichia
 Cylindrospermopsin Fever, Headache, Vomiting, Bloody diarrhea, Liver inflammation, and Kidney damage Cylindrospermopsis raciborskii, Aphanizomenon flos-aquae, Aphanizomenon gracile, Aphanizomenon ovalisporum, Umezakia natans, Anabaena bergii, Anabaena lapponica, Anabaena planctonica, Lyngbya wollei, Rhaphidiopsis curvata, and Rhaphidiopsis mediterranea
 Anatoxin-a group Tingling, burning, numbness, drowsiness, incoherent speech, salivation, respiratory paralysis leading to death* Chrysosporum (Aphanizomenon) ovalisporum, Cuspidothrix, Cylindrospermopsis, Cylindrospermum, Dolichospermum, Microcystis, Oscillatoria, Planktothrix, Phormidium, Anabaena flos-aquae, A. lemmermannii Raphidiopsis mediterranea (strain of Cylindrospermopsis raciborskii), Tychonema and Woronichinia

* Symptoms observed in animals.

Symptoms range from allergic–like reactions (e.g., rhinitis, asthma, eczema, and conjunctivitis) to flu–like reactions (skin rashes, gastroenteritis, and respiratory irritation). Allergic or irritative dermal reactions of varying severity have been reported from recreational exposures to several freshwater cyanobacterial genera such as Anabaena, Aphanizomenon, Nodularia, and Oscillatoria. Endotoxins, the blue–green pigment of the cyanotoxins (phycocyanin) and dermal toxins produced by Lyngbya and Planktothrix species have been linked to skin and eye irritation from exposure during swimming.

In addition, microcystin, anatoxin-a and saxitoxin, have been linked to gastrointestinal illness, liver disease, neurological effects, skin reactions, and possible cancer in humans. Experimental studies have demonstrated the tumor promotion activity of microcystins and nodularin. The most serious incidence occurred in 1996 at a hemodialysis clinic in Brazil when the deaths of over 70 patients receiving dialysis were attributed to exposure to microcystins which were later identified in the clinic’s water supply.

How do you treat people that have been exposed to cyanotoxins?

In the event that you do come into contact with water that is known to be contaminated with cyanotoxins, the Centers for Disease Control (CDC) recommends that you rinse off with clean, fresh water as soon as possible. Remove people from the source of exposure and give them supportive treatment. Seek medical treatment right away if you think you or someone you know might have been poisoned by cyanobacterial toxins, especially when any of the symptoms mentioned above are recognized. For more information, please visit the CDC's webpage on Harmful Algal Blooms (HABs).

How do cyanobacteria affect aquatic ecosystems?

Cyanobacterial blooms may cause detrimental effects on aquatic ecosystems. High biomass blooms, whether of toxic or nontoxic species, can accumulate as thick scums and mats, which decompose causing excessive oxygen consumption (hypoxia), which leads to an increased mortality rate in local fish, shellfish, invertebrate, and plant populations as a result of low oxygen levels (hypoxia). The blooms may also affect benthic flora and fauna due to decreased light penetration. Toxic blooms from some cyanobacteria genera (e.g., Anabaena circinalis, Aphanizomenon flosaquae, Cylindrospermopsis raciborskii, and Microcystis aeruginosa) may lead to inhibition of other phytoplankton and suppression of zooplankton grazing, leading to reduced growth and reproductive rates and changes in community structure and composition.

What other impacts do cyanobacteria have on their environment?

In addition to the production of toxins, cyanobacteria have often been associated in drinking water with taste and odor problems. Algal scums can be quickly broken by wave action and redispersed by wind mixing. In shallow bays, scums may take a long time to disperse and cells may disintegrate and die. Dying and lysing cells release their contents (toxins) into the water and are subject to rapid putrefaction of the material. Blooms produce a variety of odor and taste compounds, such as geosmin and 2–methylisoborneol (MIB), which are not toxic but are a nuisance to the public. The cyanobacterial genera that are known to produce geosmin are Anabaena, Aphanizomenon, Lyngbya, Microcystis, Oscillatoria, Phormidium, Schizothrix and Symploca; however, taste and odor issues are complex and are not solely associated with the presence of cyanobacteria. Many actinomycetes bacteria such as Actinomyces and Streptomyces species, aquatic fungi and myxobacteria can also produce these compounds.

More Information

US EPA Health Effects Support Document for the Cyanobacterial Toxin Anatoxin-a
US EPA Health Effects Support Document for the Cyanobacterial Microcystins Toxins
US EPA Health Effects Support Document for the Cyanobacterial Toxin Cylindrospermopsin
US EPA Drinking Water Health Advisory for the Cyanobacterial Toxin Cylindrospermopsin
US EPA Drinking Water Health Advisory for the Cyanobacterial Microcystins Toxins
Interagency, International Symposium on Cyanobacterial Harmful Algal Blooms
US EPA Harmful Algal Blooms and Seafood Safety
US EPA IRIS Toxicological Reviews for Microcystins, Anatoxin-a, and Cylindrospermopsin
WHO Cyanobacterial toxins: Microcystin-LR in Drinking-water
WHO Water Related Diseases: Cyanobacterial Toxins
Health Canada Blue-Green Algae (Cyanobacteria) and their Toxins
Presentations EPA Webinar Human Health Risks Associated with Cyanobacteria and Cyanotoxins Exposure, May 22, 2013
Presentations EPA Webinar Human Health Risks Associated with Cyanobacteria and Cyanotoxins Exposure, May 23, 2013

For comments, feedback or additional information, please contact Lesley D'Anglada (, Project Manager, at 202-566-1125.

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